Previously, I talked about the basic facts behind batteries. Now, let's talk about how to put them into practice...
NiMH and NiCad batteries both like to go through a normal charging cycle, so you don't want to take 10 shots and then shove them back in the charger. There's some debate and religious ritual involved here and very little data. It should be OK to just use them until they reach 1 volt per cell and then charge them again. If you want to be sure, you can have your charger discharge them all the way down, but that also means that you are wearing out the battery without actually using it to power something, so it won't last as long.
All batteries are damaged if you charge them too low. Be careful about trying to get one last shot of them -- the "low battery" light may not trigger until after you've reached the point of damage. With Lithium-Ion batteries, there's absolutely no benefit in discharging them other than use.
Under no account should you try to discharge a battery all the way by leaving your device on.
Your battery may survive a few deep discharges, but every time damages the battery. Eventually it just won't work. Or if you discharge it too deep too quickly, it'll just blow up.
If your device takes 4 AA batteries, keep them in a set. If you have one battery that has less capacity than the others, the weakest battery will get fried before the device stops working. The easiest way to avoid this is to keep batteries of more or less the same capacity and charge properties in a set. I usually write on the battery a letter with a marker, so I can track this.
There's pretty much three grades of chargers for NiMH batteries.
The first category are the cheap overnight chargers. Cheap chargers will "gang up" two batteries and charge all batteries with a "trickle" charge that won't damage the battery too much. They may not accurately sense when a battery has been fully charged (or not bother sensing it at all) so if you put your batteries in one for a week or more, it'll start damaging the cells. In theory, they should be able to equalize the charge of a set of batteries, but in practice they don't always do it properly.
The second category are the fast chargers that still "gang up" batteries into sets. They will charge faster, but aren't always that great for the batteries. They don't always use the best parameters to detect if the battery has finished charging, so they can overcharge it. In theory, they are supposed to drop into "trickle" charge mode and try to get the cells equalized, but that doesn't always work right either. I am not impressed with these chargers at all.
The third category are the good ones. They charge each cell individually. They can also discharge each of the batteries down to the safe minimum and then charge each battery up to the safe maximum. They often times also have "test" modes to let you sort out a batch of unsorted batteries into sets and "refresh" modes that may or may not make your batteries abused by cheap chargers act better.
The thing is, I've seen cheap chargers that are in the third category but cost less than a fancy name-brand charger in the second category.
Now, manufacturers really want to sell you the second category and have you think that they sold you the third category. So they'll put an indicator light on each slot, but they'll still batch them. The real thing is that your batteries will all finish charging at different times and that you can put them in a cell at a time.
There's some noises made about fancy charger features that will zap your batteries in new and special ways, but I suspect the biggest advantage is just being able to charge batteries individually and to be able to charge all different sizes. There was also some noises about super-fast chargers that would charge a set of batteries in 15 minutes or so. The trick was to put a little chip inside of the battery that kills the charging when the battery is full. The problem is that you can't take any random set of NiMH batteries and put them in a 15 minute charger and have it work, so I haven't seen much of them recently.
Lithium batteries have basically no wear from charging and a fairly high risk of being damaged by being discharged all the way down, so just toss 'em in the charger after you are done shooting.
The biggest, best feature of Lithium-Ion batteries is that they like it when you do this.
The only thing you can do to make Lithium-Ion batteries last longer is to keep them cool.
In theory, one could get longer life out of them by storing them at a reduced charge in the fridge, but I'm not sure how that is supposed to help, given that the battery still died over time. But clearly not keeping them in hot places is a good idea.
Since the clock-of-battery-life starts ticking when the battery is made and you can only change that so much, it is probably better to buy two batteries now, two batteries next year, and two batteries the year after than to buy six batteries now.
This keeps biting me.
First, if you take the batteries out, you don't need to worry about the power switch being left on.
Second, if you leave the batteries in the device and leave it alone for a while, it might leak the contents. There's a few ways this can happen, but it's always nasty.
It's hard to do this with AA or AAA batteries because there's a connection on each side. But a battery can and will blow up if you do it. LiIon batteries tend to have contact surfaces clustered on one end, so if you keep it in a pocket with your keys, you can have a minor blow-up.
Battery covers and cases are a good thing. If for no other reason than making for a neat and clean camera bag.
Two other things to keep in mind:
First, don't accidentally put a magazine atop your battery charger after moving it from the usual place you keep it after going on a weekend getaway with the wife. You will spend a long time hunting for the damn thing.
Second, if you have spare batteries, bring some.